The use of heavy media separation is well known in mineral preparation and the like, e.g., coal preparation. The process comprises introducing finely divided particles of high magnetic susceptibility (EG magnetite or ferrosilicon) into water to form a slurry, adjusting the amount of magnetite or ferrosilicon so that the slurry has a desired specific gravity and then introducing the mineral into the slurry. A separation is achieved between those mineral particles which have a specific gravity less than the specific gravity of the slurry and which float across, and those mineral particles which have a higher specific gravity than the slurry and which sink. If the mineral is coal, the magnetite slurry may be adjusted to have a specific gravity in the range of 1.35 to 1.50, for example. Pieces which have a specific gravity of less than 1.35 will float and may be assumed to be high quality coal without greatly reduced ash or sulfur content. Pieces which sink at a specific gravity of 1.50 may be considered to be predominantly refuse. Pieces between 1.35 and 1.50 specific gravity may be considered to be of intermediate quality.
The cost of magnetite for heavy media separation is significant and it is desirable to recover the magnetite to the greatest possible extent. It has been found that the presence of magnetite particles smaller than 5 microns is especially helpful in providing a stable process which leads to an increased yield. Improved recoveries of clean coal, for example, may be expected to be in the range of 2 to 5% if the slurry includes a certain minimum proportion of -5 micron particles.
In the operation of a heavy media separation plant separating coal, the float coal and sink coal from the heavy media vessels carry significant quantities of magnetite from the vessels by surface adhesion. The coal is passed over screens where it is washed to remove the magnetite, and it is then moved to storage silos or the like. The magnetite and wash water are passed through magnetic separators where the magnetite is removed to the greatest extent possible and is returned to the heavy media vessel. The use of magnetic separators is well known and numerous designs have been tried.